The present invention relates to a MOS--type semiconductor device with a current detection function, in particular, to an improvement in a detection voltage dependency of a current detection characteristics in an IGBT, a power MOS transistor and the like.
FIG. 10A is a cross-sectional view of a conventional semiconductor device including N-channel type planar gate MOSFETs, in particular, of a boundary portion thereof between the main current cell region and current detecting cell region. FIG. 10B is a plan view showing the pattern of the boundary portion. The cross-sectional view of FIG. 10A is taken along line XA--XA of FIG. 10B.
As shown in FIGS. 10A and 10B, the semiconductor device comprises an N-type substrate 10, an N-type drift region 1 formed on the upper surface of the substrate 10, and a drain electrode 9 formed on the back-side surface of the substrate 10. Formed in the drift region 1 are a number of P-type base regions 2 in which N-type source regions 3 are provided. Gate insulating films 4 are formed on the drift region 1, base region 2 and source region 3. Gate electrodes 5 are formed on the gate insulating films 4 and covered by interlayered insulating films 6.
A main current electrode 7 is provided on the surface of the main current cell region, extending into openings 12 between the gate structures of the main current cell region. A current detecting electrode 8 is provided on the surface of the current detecting cell region, extending into an opening 11 between the gate structures of the current detecting cell region. The base regions 2 and the source regions 3 of the main current cells are in contact with the main current electrode 7 via the openings 12. The base region 2 and the source regions 3 in the current detecting cell, are in contact with the current detecting electrode 8 via the opening 11.
In each of the openings 12 of the main current cells, the ratio of the area of the contact portion of the base region 2 to the main current electrode 7 to the total area of the contact portions of the source regions 3 to the main current electrode 7 is approximately 6:4. The same is true for the opening 11 of the current detecting cell. That is, in the opening 11 of the current detecting cell, the ratio of the area of the contact portion of the base region 2 to the current detecting electrode 8 to the total area of the contact portions of the source regions 3 to the current detecting electrode 8 is approximately 6:4.
For the current detection, an external resistor (not shown) is connected between the main current electrode 7 and the current detecting electrode 8 to detect a potential difference across the resistor. The current value is obtained from the quotient of the potential difference divided by the resistance value of the external resistor.
In general, the main current cells are provided adjacent to the current detecting cell on a semiconductor chip. A MOS gate provided between the current detecting cell and main current cells is opened in a current detection state. Thus, the current detecting cell is electrically connected to the main current cells. Inevitably, a portion of current which will otherwise flow through the external resistor flows into the main current cells through a parasitic resistor formed in the drift region 1.
In addition to the above disadvantage, the conventional device has the following further disadvantage:
The parasitic resistor is a bulk resistor of silicon and thus has a temperature dependency characteristics. The parasitic resistance increases as the temperature rises. Thus, current flowing through the external resistor increases as the semiconductor device operates, thus generating heat. Consequently, the potential difference across the external resistor (hereinafter referred to as "detection voltage") increases, and the current can no longer be accurately detected.
To reduce the disadvantage due to the temperature characteristics of the parasitic resistance, it suffices to increase the parasitic resistance so that the parasitic resistance is larger than the external resistance.
In order to increase the parasitic resistance effectively, the distance between the main current cell region and the current detecting cell region may be increased. However, if the distances between the base regions 2 of the main current cells and the base region 2 of the current detecting cell are increased, the curvature of each of depletion layers is increased. As a result, an electric field concentrates between those base regions, lowering the breakdown voltage. Therefore, in the conventional device, the distance between the main current cell region and the current detecting cell region must be long enough to electrically isolate the main current cell region from the current detecting cell region, but should not be so long as to lower the breakdown voltage.
For the above reasons, it is difficult for the conventional device to reduce the influence of the temperature characteristics of the parasitic resistance, and to prevent lowering of the breakdown voltage.
Furthermore, when the detection voltage exceeds 0.6V, a forward bias voltage is applied to the PN junction between the P-type base region 2 of the current detecting cell and N-type drift region 1. Holes flow from the base region 2 into the drift region 1. The resistance of the drift region 1 rapidly decreases. So does the parasitic resistance between the detecting electrode 8 and the main current electrode 7.
FIG. 8 also shows the relationship between the detection voltage and the parasitic resistance in the conventional semiconductor device and a semiconductor device of the present invention. As is clearly understood from FIG. 8, the parasitic resistance rapidly decreases when the voltage exceeds 0.5V in the conventional device.
The current which is to be flown through the external resistor inevitably flows through the parasitic resistance. The ratio of current flowing through the detecting cell (hereinafter referred to as "detection current") to current flowing through the main current cells (hereinafter referred to as "main current") therefore decreases. FIG. 9 also represents the relationship between the detected voltage and the ratio of the main current to the detection current in the conventional semiconductor device and a semiconductor device of the present invention. As is seen from FIG. 9, the ratio rapidly increases when the detection voltage exceeds 0.5V in the conventional semiconductor device.
Thus, the detection current varies due to variation of the detection voltage. Therefore, it is difficult to design a current detecting circuit. Furthermore, the main current greatly varies in the current detection operation, and thus the device may malfunction.
In general, semiconductor devices for use in current detection are of bipolar type. In a bipolar-type semiconductor device, the detection voltage is set to 0.5 to 0.6V in order to detect a current. However, if an MOS-type semiconductor device is used to detect a current, the detection voltage needs to be set to about 1V, due to the difficulty in control of the threshold voltage of the MOS transistors. As shown in FIG. 8, the parasitic resistance is considerably low when the detection voltage is about 1V. Thus, the parasitic resistance can hardly be set at a value greater than the external resistance.